Loop detection mechanism

The device detects loops by sending detection frames and then checking whether these frames return to any port on the device. If they do, the device considers that the port is on a looped link.

Loop detection usually works within a VLAN. If a detection frame is returned with a VLAN tag different from the one it was sent out with, an inter-VLAN loop has occurred. To remove the loop, examine the QinQ configuration for incorrect settings. For more information about QinQ, see "Configuring QinQ.".

The Ethernet frame header for loop detection contains the following fields:

• DMAC—Destination MAC address of the frame, which is the multicast MAC address 010F-E200-0007. When a loop detection-enabled device receives a frame with this destination MAC address, it sends the frame to the CPU and floods the frame in the VLAN from which the frame was originally received.

• SMAC—Source MAC address of the frame, which is the bridge MAC address of the sending device.

• TPID—Type of the VLAN tag, with the value of 0x8100.

• TCI—Information of the VLAN tag, including the priority and VLAN ID.

• Type—Protocol type, with the value of 0x8918.

The inner frame header for loop detection contains the following fields:

• Code—Protocol sub-type, which is 0x0001, indicating the loop detection protocol.

• Version—Protocol version, which is always 0x0000.

• Length—Length of the frame. The value includes the inner header, but excludes the Ethernet header.

• Reserved—This field is reserved.

Frames for loop detection are encapsulated as TLV triplets.